Purification and characterization of an antifungal thaumatin-like protein from Cassia didymobotrya cell culture

A 23-kDa antifungal thaumatin-like protein was isolated and purified from Cassia didymobotrya (Fres.) cell cultures for the first time. The protein was secreted in the culture medium, but it could be also isolated after elution of whole cells with a 0.5 M CaCl2 solution. Treatment of the cells with laminarin oligosaccharides or salicylic acid, but not with NaCl, resulted in enhancement of expression of the protein. A rapid purification protocol was used based on cationic exchange chromatography. The protein, with a highly basic character (pI 10), has an exact molecular mass of 23034 Da, as determined by MALDI-ToF mass spectrometry analysis. N-terminal sequencing of the intact polypeptide and the sequencing of two internal tryptic peptides indicated significant identity with other thaumatin-like proteins (TLP). The protein exerted antifungal activity towards some Candida species showing EC50 values comparable to those of other antifungal TLPs. The collected data lead to classify this TLP as a new PR-5 protein.